1. Field of the Invention
The invention relates to the field of combustion technology. It relates to a burner of the double-cone design, in which gaseous fuel is fed to the combustion-air flow before it flows into the interior space of the burner.
2. Discussion of Background
U.S. Pat. No. 4,932,861 to Keller et al. discloses the basic construction of a burner of the double-cone design, to which the invention relates. This burner essentially comprises hollow sectional cone bodies mounted adjacent one another to defining a conical interior space that widens along a flow direction having tangential air-inlet slots and feeds for gaseous and liquid fuels. The center axes of the hollow sectional cone bodies are mutually laterally offset from one another longitudinal direction. A fuel nozzle is placed at the burner head in the conical interior space formed by the sectional cone bodies. Via gas injectors arranged along the inlet slots, the gaseous fuel is fed to the combustion-air flow prior to its inflow into the burner interior space. The fuel/air mixture therefore forms directly at the end of the tangential air-inlet slots. The air-inlet plane and the gas-inlet plane (perforation plane) therefore coincide in this known prior art.
The increase in swirl along the cone axis, in combination with the sudden widening in cross section at the burner outlet, leads to the formation of a backflow zone downstream of the burner outlet on the burner axis, which backflow zone stabilizes the flame. The ignition of the flame is not initiated until the stagnation point of the backflow zone.
In this known prior art, the last gas injectors along the air-inlet slots lie very close to the burner outlet and thus also lie in the vicinity of the flame. The length of the premix section is therefore very short at these points, so that the fuel which is injected from these last downstream nozzles is not able to mix very well with the inflowing combusting air. Local zones having a rich fuel/air mixture develop due to the poor premixing of the fuel with air, which rich fuel/air mixture leads to higher flame temperatures and thus also to higher NOx values. Furthermore, the additional loading becomes so high for the burner front in these regions that overheating occurs and the material has to be protected there by an expensive zirconium coating.
If it is desired to lengthen the premix section along the burner axis in order to reduce the NOx emissions, a complicated transition piece is necessary for this purpose between the burner and the following part, for example a tube arranged in front of the combustion chamber. The flow zone produced downstream by the burner results in problems with the axial velocity in the downstream part, either at the margin or in the center. This leads to flashbacks, so that the burner cannot be operated in this manner.